# Pressure drops in tank of gas

1. May 25, 2016

### prehisto

1. The problem statement, all variables and given/known data
Pressure drops in welders tank of oxygen gas from p1=150atm to p2=120atm .
How much of the gas will be used ?
2. Relevant equations

3. The attempt at a solution
In my mind the simplest way of looking at this problem is to consider the process isothermal.
But then the volume of the gas increases when pressure drops (Ideal gas law). This contradicts the question.

Since I do not know the temperatures before and after, I dont know what to do. Im looking for different view at this problem, some help?

2. May 25, 2016

### Simon Bridge

If you want to treat the gas as an ideal gas then state the ideal gas law.
Looking at the law, which of the variables is constant and which variable? List them.
i.e. does the volume of the tank change?

3. May 25, 2016

### Guneykan Ozgul

Volume of the gas does not change if the tank is not elastic (I think not in this question) since the gas will always fill the whole tank.

4. May 25, 2016

### prehisto

OK, I Think it is logical to assume that Volume of tank does not change. Now I can rethink my solution.
$P_1V=n_1RT_1$ and $P_2V=n_2RT_2$
$\frac {n_1RT_1} {P_1} = \frac {n_2RT_2} {P_2}$
$\frac {n_2} {n_1}=\frac {T_2P_1} {T_1P_2}$

But It seems that I need the temperature before and after, which i do not have. And to assume that T=const seems to be to only way, but it also seems not logical.

5. May 25, 2016

### Staff: Mentor

You're supposed to assume that the tank is not insulated, so that the final and initial temperatures are equal to room temperature.